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The ignition of the Delta IV Heavy rocket is perhaps the most visually striking liftoff you’ll ever see — the rocket seemingly burns itself up on the launchpad before it heads to space. Now, the very last Delta IV Heavy ever is on the launchpad.

Liftoff is scheduled for 1:40 p.m. Eastern time from Cape Canaveral, Fla., although winds and clouds could keep the rocket on the ground for another day or a bit longer. Forecasts give only a 30 percent chance of favorable weather. Conditions are expected to be better during backup launch opportunities.

“A bittersweet moment for us,” Tory Bruno, the chief executive of United Launch Alliance, the maker of the Delta IV Heavy, said during a news conference on Wednesday ahead of the flight, which is carrying a secret spy satellite for the National Reconnaissance Office. “This is such an amazing piece of technology. Twenty-three stories tall. Half a million gallons of propellant. A quarter million pounds of thrust.”

When it does launch, it will look as if it is catching on fire, with flames racing up the sides. That is by design.

The Delta IV Heavy burns ultracold liquid hydrogen, which is a high-performance fuel. In the final part of the countdown, to cool down the engines and prevent a sudden temperature shock that could cause cracks, liquid hydrogen starts flowing through the engine into the flame trench.

But when the hydrogen warms above its boiling temperature of minus 423.2 degrees Fahrenheit, it turns into a gas. Hydrogen is lighter than air and rises upward. When the engines ignite, so does that cloud of hydrogen — like a space-age Hindenburg.

“A very dramatic effect,” Mr. Bruno said.

The rocket designers of course took this into account and applied sufficient insulation to the boosters to keep the rocket from actually burning up. The orange shade of that exterior takes on a burned-marshmallow sheen as the rocket leaves the Earth.

“And away she goes,” Mr. Bruno said.


Photographs by United Launch Alliance. Mobile photo illustration by Antonio de Luca.

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Credit: NYTimes.com

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